Click the date and time in the menu bar or swipe left on the trackpad to open Notification Center
Scroll to the bottom and click Edit Widgets
Search for a widget or browse the widget list
Click a widget to add it
Choose the widget size if available
Drag the widget to the desired location
Click Done
To add widgets to the desktop, right-click the desktop and select Edit Widgets
Browse or search for a widget
Click or drag the widget onto the desktop
Click Done